Fix trajectory evaluation column name in evaluating_adk_agent.ipynb

Rename 'predicted_trajectory' to 'reference_trajectory' in eval_data dictionary (cell 28) to align with Vertex AI Gen AI Evaluation service requirements.

This fixes the 400 error: 'Required field is not set' when running trajectory evaluation metrics (trajectory_exact_match, etc.).

The evaluation dataset should use 'reference_trajectory' for ground truth data, while 'predicted_trajectory' is generated automatically by the agent during evaluation.

Author: prabha-git